The mosque’s walls had gone black. In the villages near the Sunda Strait, residents were out with brooms in the first days of September, sweeping the grey dust from the floors of their houses and off the blackened walls of a nearby mosque, work that Reuters photographers found them doing while the volcano was still going. At night, the lava from Anak Krakatau lit the sky red.
Mount Anak Krakatau began erupting on the Saturday, throwing up a lava fountain, and residents in several parts of the country reported booming sounds and tremors, according to Indonesia’s geology agency. At half past one on Sunday morning, after ash was detected in the airspace around it, Soekarno-Hatta International Airport suspended flight operations. The transport ministry explained the decision in the flat language of procedure: the latest paper test showed positive results indicating the spread of volcanic ash in the airport area, so the closure was extended to half past nine that morning. By the ministry’s count, 209 flights were affected and more than 22,000 passengers, with services to and from Singapore hit hardest, and routes to Doha, Sydney and Kuala Lumpur disrupted too. Local airport authorities counted at least sixteen domestic flights to Bali cancelled.
The reopening did not hold. By Monday morning seven Indonesian airports, two of them serving Jakarta, were still closed because of the ash, the transport minister said. The ash had blanketed the capital and several surrounding areas since the eruption began late on Friday, and the airports were shut for safety reasons.
Safety is a word that carries a specific memory in that airspace. On the night of 24 June 1982, a British Airways Boeing 747 named City of Edinburgh, flying from Kuala Lumpur towards Perth, ran into ash from the eruption of Mount Galunggung about 110 miles south-east of Jakarta. Because the cloud was dry, the weather radar, built to find moisture, showed clear skies. Captain Eric Moody was on his way to the lavatory when the crew noticed a glow on the windscreen like St Elmo’s fire. Smoke began collecting in the cabin, first mistaken for cigarette smoke, then smelling of sulphur. Passengers with a view of the engines saw them shining an unusual bright blue, light coming forward through the fan blades in a strobe. At 13:42 UTC the number four engine surged and flamed out. Within a minute the other three had gone too, and the flight engineer said what everyone was thinking: “I don’t believe it—all four engines have failed!”
What followed is the reason ash advisories exist. The crew worked out they could glide for twenty-three minutes and 91 nautical miles from 37,000 feet, and broadcast a mayday that Jakarta control misheard as a single engine failure until a Garuda Indonesia flight relayed it. Moody told the cabin: “Ladies and gentlemen, this is your captain speaking. We have a small problem. All four engines have stopped. We are doing our damnedest to get them going again. I trust you are not in too much distress.” Many passengers wrote notes to relatives. At 13,500 feet, close to the altitude at which they would have had to turn out to sea and try a ditching no one had ever attempted in a 747, engine four caught, then three, then one and two. The windscreen had been sandblasted opaque, so the approach into Halim Perdanakusuma was flown almost entirely on instruments, the first officer calling out heights against the airport’s distance-measuring equipment to build a glide slope out of nothing. Moody called it a bit like negotiating one’s way up a badger’s arse. At the bottom of the steps the flight engineer knelt and kissed the ground; asked why, he said the Pope does it, and Moody replied that the Pope flies Alitalia.
The investigation found the mechanism. Ash starved the engines of oxygen, sandblasted the windscreen and landing-light covers, and clogged the machinery; inside the combustion chambers it melted and stuck to the interior of the power plant. Laboratory work has since put numbers on that. Jet engines run at 1,200 to 2,000 degrees Celsius, hot enough to melt ingested ash and make it adhere to turbine parts, and because chemical composition varies widely from volcano to volcano, so does the temperature at which a given ash turns liquid. One group of researchers heated samples to parameterise the behaviour and build a model predicting when ash will melt and stick across the global compositional range. Their experiments also demolished a convenient shortcut: using sand or dust as a proxy is wholly inadequate, overestimating the sticking temperature and so badly understating the thermal hazard.
The other reason airports close, rather than route around the plume, is that nobody can say with confidence where the plume is. Dispersion forecasts are acutely sensitive to the physical characteristics assumed for the particles, because fall velocity depends on diameter. Long-range transport models consider particles from 0.1 to 100 micrometres across; a 100 micrometre grain falls more than five orders of magnitude faster than a 0.1 micrometre one, and a 30 micrometre grain falls 88 per cent more slowly and travels up to five times further than a 100 micrometre grain. The size distribution at the source, which the model needs at the start, is hard to know, and clumping of particles is not explicitly modelled in operational systems because the computing cost is too high.
Europe learned what that uncertainty costs in April 2010. On 14 April, meltwater from the Eyjafjallajökull glacier met hot magma and an explosive eruption pushed unusually fine ash into the jet stream, where it spread quickly over the continent, and air traffic was grounded for days. On the Thursday, between 5,000 and 6,000 of Europe’s 28,000 daily flights were cancelled; at Heathrow’s Terminal 4 every check-in counter was closed and the boards showed nothing but cancellations. The next day Eurocontrol expected 11,000 flights instead of the usual 28,000, around sixty Asia-Europe services were called off, stranded passengers at Frankfurt were handed camp beds, and in Copenhagen Queen Margrethe’s seventieth birthday guest list shrank as the flight bans kept dignitaries away, including Spain’s King Juan Carlos and Iceland’s own president. Jerry O’Callaghan, fifty, from Cork, coming home from a holiday in America, had been about to board when the flight was cancelled. They should have better equipment, he said. It’s only a load of ash.
The volcano now dusting Jakarta has form of a different order. Krakatoa’s explosions of 26 and 27 August 1883 destroyed more than seventy per cent of the island, the third blast of that morning remaining the loudest sound known to history, audible 3,110 kilometres away in Perth and 4,800 kilometres away near Mauritius, with at least 36,417 deaths attributed to the eruption and its tsunamis. The cone that emerged from that caldera first appeared in 1927 and stood permanently above the sea from 11 August 1930, and through September 2018 its summit was rising by seven to nine metres a year. On 22 December 2018 its south-western flank slid away on a dipping décollement, two minutes after a small earthquake, decapitating the cone and sending a tsunami that killed 430 people.
In 2010, asked how long the disruption would last, an Icelandic geophysicist named Einar Kjartansson gave the only honest answer available then and now. The ash would keep coming for days or weeks, he said, but whether it wrecked anyone’s travel was a matter of weather: “It depends how the wind carries the ash.”